1. Field of the Invention
The present invention relates to a zoom lens for projection especially appropriate for projection onto a large screen in a movie theater or the like, and to a projection-type display apparatus.
2. Description of the Related Art
Conventionally, projector apparatuses (projection-type display apparatuses) using light valves, such as a liquid crystal display device and a DMD (Digital Micromirror Device: Registered Trademark) display device, and which project images by using lenses having relatively long back focus, were widely used.
In recent years, projector apparatuses that can project higher definition images appropriate for large screens gradually started to be used in movie theaters or the like.
The projector apparatuses for such a purpose adopt a reflective liquid crystal display device method or a DMD three panel method. Therefore, projection lenses used in the projector apparatuses need to have longer back focus, and excellent telecentricity similar to other kinds of projector apparatus.
Meanwhile, as a zoom lens for projection used in such a projector apparatus, a zoom lens that is focused by moving a first lens group on the magnification side of the zoom lens is known. However, since the outer diameter of the first lens group is generally large, the weight of the first lens group is heavy. Therefore, when the first lens group is moved to focus the zoom lens, a load on a mechanism for focusing tends to be large. As an attempt to solve such problems, a zoom lens in which a first lens group is divided into a first-lens-group front group and a first-lens-group rear group, arranged in this order from the magnification side, is known (please refer to U.S. Pat. No. 7,576,921 (Patent Document 1) and Japanese Unexamined Patent Publication No. 2004-226803 (Patent Document 2)). In the zoom lens, focusing is performed by moving only the first-lens-group rear group in the direction of an optical axis, while the first-lens-group front group is fixed. Further, a zoom lens in which focusing is performed by moving, in the direction of an optical axis, a lens group (third lens group, for example) that is located on the reduction side of the first lens group and that moves while magnification is changed is known (please refer to Japanese Unexamined Patent Publication No. 2006-162700 (Patent Document 3)).
However, in the zoom lens disclosed in Patent Document 1, the first-lens-group rear group that is moved in focusing is composed of a lens. Therefore, there is a problem that chromatic aberrations caused by focusing are too large.
Further, in the zoom lens disclosed in Patent Document 2, the first-lens-group rear group that is moved in focusing is composed of four lenses, namely, second through fifth lenses in the first lens group. Therefore, a load on the mechanism for focusing is not reduced, compared with the case of moving the whole first lens group.
Further, in the zoom lens disclosed in Patent Document 3, focusing is performed by moving the third lens group, which moves while magnification is changed. Therefore, a lens movement mechanism for focusing, which operates separately from a lens movement mechanism for zooming, needs to be provided in addition to the lens movement mechanism for zooming. Hence, the lens movement mechanisms become enormously complex.